Recording gain regulation system



March 3, 1936. R. RECHNITZERW 2,032,436

RECORDING GAIN REGULATION SYSTEM Filed June 22, 1933 g/ceo /m/vf M/C p Y ,Q/-. AA/PA/F/f/Z 1 25602052 2/ TWA/{541F759 ear/Hag i5 An l/HEB.

INVENTOR 2000M efcl/larzfe ATTORNEY Patented Mar. 3, 1936 RECORDING GAIN REGULATION SYSTEM Rudolf Rechnitzcr, Berlin, Germany,

Teleiunken Gesellschai' assignor to t fur Drahtlose Telegraphic m. b. 11., Berlin, Germany, a corporation of Germany Application June 22,

In Germany 1933, Serial No. 677,026

July 28, 1932 I 4 Claims. (01. 179 100.1)

The present invention relates to a method for undistorted recording and reproducing of acoustic actions.

In recording and reproducing acoustic actions- 5 disturbances often are occasioned because of the fact that the amplitudes of the oscillations to be recorded, or reproduced, tend to overmodulate some of the parts in the transmission means. Non-linear distortions which thus arise result in combination sounds which undesirably cause vitiations 0f the timber, and this imparts hoarseness to the sounds reproduced.

In order to obviate these drawbacks it has been suggested in the prior art to control the gain of the electric amplifier in dependence upon the amplitude of the audio frequency in such a manner that, in the presence of large amplitudes, the gain is diminished. Changes in dynamics possibly occasioned thereby are subsequently compensated 2 whenever this should be required, in the following stages. Such an audio amplitude regulation, for instance, by the intermediary of grid potential shifts, results in this drawback that the working point of the tube to be controlled comes to lie inside the curved part of the characteristic, otherwise a variation in the transmission measure would be impossible. However, when working upon the curved part of the characteristic, nonlinear distortions arainevitable so that a regula 0 tion oi the said sort wouldbe" permissible only to a limited extent. i\

Now, according to the present invention, the inconvenience before referred to is avoided by that the audio impulses which arrive from the microphone, the acoustic pick-up, a photocell, etc. are employed for the modulation of a radio transmitter, the amplitude of which is subject to control in dependence upon the amplitude of the audio energy. The distortions incidentally occasioned in the radio oscillations play no part at all inasmuch as the audio oscillations are not impaired incidentally. By shifting the grid bias voltage of the radio'amplifier tube it is possible to regulate the radio energy and after rectification also the audio energy as regards their amplitudes without audio non-linear distortions tending to impair reproduction.

The control device, for instance, could consist of a rectifier tube for the audio energy which is fed by the recorded audio energy. The tube works with a negative biasing voltage so that in normal state no plate current will flow. If, then, the amplitude rises beyond a certain pre-arranged level, flow of current across the tube is initiated. This current sets up a fall of potential across a resistance which is used for increasing the negative grid voltage of the radio amplifier. Thus the gain is automatically restored to the admissible measure.

Fig. 1 illustrates the operation of the control 5 tube. The negative grid biasing voltage of the control tube is so chosen that the working point A is spaced apart from point B of the characteristic of the control tube where the plate current begins to flow in accordance with the size of the maxi- 1o mum audio amplitude N transmissible under con-' ditions free from distortion. As soon as the amplitudes of the audio energy begin to exceed the amount AB as represented, for instance, by the dotted curves, the plate current starts flowing 15 in accordancewith the increase in the amplitudes. This plate current is made to flow through a re- 1 sistance, and the fall of potential occasioned across the terminals of the latter is used for increasing the negative grid voltage of the radio 20 tube.

According to another object-of this invention, the arrangement could further be equipped with regulator means for the lowest occurring amplitudes. For this purpose is employed another 25 rectifier tube which works approximately about the middle of the rectilinear part of the characteristic. Under normal ccnditimis the amplitudes of the audio energy are so large modulation. will always happen at points beyond the top knee, 30 with the result that the plate'direct c t of this tube remains substantially constant. amplitudefallsbelowacertainpreed measure, the plate current will drop correspondingly and thereby diminish the fall of voltage across 35 another resistance which, in turn f es the radio tube with grid bias voltage-in such a way that the gain of the transmission means is raised.

- 2 illustrates the operation of this arrangement. Detailed explanations will not be needed in the light of what precedes. A denotes the working point, M the normal audio amplitude, and m the minimum amplitude. By changing the working points of these two tubes, the amplitude range can be altered at will.

Fig. 3 shows schematically one embodiment of the basic idea of the invention as applied, for instance, to an arrangement suited for phonograph record recording. In equipment of this kind, the gain is usually controlled and adjusted by an operator, while in the embodiment here disclosed the said monitoring work is eflfected automatically. The audio impulses coming from the microphone l are fed into the microphone amplifier 2. Part of this energy serves for the modulation of the radio transmitter I.

The frequency of the said radio transmitter 3 may be chosen ad libitum. Another part of the said impulses is fed to the rectifier tube 4 which serves for the regulation of the maximum amplitude; a third portion is supplied to the tube 5 by the agency of which the minimum or smallest amplitudes are controlled. (See Figs. 1 and 2.) The plate voltage of the tubes 4 and 5 serve for the control of the grid bias potentials of the radio amplifier 6. Above the said radio transmitter 6 are disposed the rectifier 1, the audio part 8 and the sound recorder device 9.

The same scheme could analogously be employed in recording and reproducing apparatus for sound-film purposes, or else for loud-speaker equipment.

What is claimed is:

1. A method which. includes the following steps: producing electrical waves, modulating the waves with energy of an audible frequency, amplifying the modulated waves, rectifying a portion of the modulation energy to produce a direct current voltage varying in intensity therewith, rectifying another portion of the modulation energy to produce a second varying direct current voltage, and regulating the amplification of-the modulated waves with both direct current voltages.

2. A method of regulating an electrical wave transmission system which includes modulating mam electrical waves with audio frequency waves, anrh plifying the modulated waves, rectifying the amplified waves, rectifying a portion of the modulation waves and utilizing the. rectified current for decreasing the modulated waves amplification when the modulation waves increase above a desired amplitude, and rectifying another portion of the modulation waves and utilizing the rectified current for increasing the said amplification when the modulated waves decrease below a desired amplitude. I

3. In an electrical system, a microphone, a source of carrier waves, an amplifier arranged to receive the outputs of said source and microphone, a rectifier connected to said microphone for producing. a direct voltage varying in the same sense as the amplitude of the microphone output waves, a second rectifier connected to the microphone for producing a direct voltage also varying with the amplitude of the microphone waves, one of said rectifiers being connected to said amplifier to decrease the gain of the latter when the microphone waves rise above a desired amplitude level, the other rectifier being connected to the amplifier to increase the gain thereof when the microphone waves drop below a de-' sired amplitude level.

4. In a system as defined in claim 3, a rectifier connected to the amplifier output, an audio amplifier connected to the last rectifier, and a recorder connected to the audio amplifier.

RUDOLF RECHNITZER. 

